Spectrographic analysis particularly in the near infrared range is a powerful tool in analyzing substances and their properties. Infrared spectrographic analyzing instruments are used to measure the constituents of agricultural products and the composition and property of chemical substances. Spectrographic analysis instruments operating in the visible range also have important uses, for example, in analyzing and matching colors.
Typically, a spectrographic instrument employs a source of light having a wide spectral band extending over the spectral range of operation of the instrument. Light from the source is dispersed by the instrument into its spectral components and the dispersed spectral light is used in the analysis. The instrument of the type to which the present invention is directed employs an optical grating which is oscillated or rotated. Light from the source irradiates the optical grating through an entrance slit and the optical grating disperses the light into its spectral components toward an exit slit. A narrow wavelength band of light will pass through the exit slit. As the grating rotates or oscillates, the center wavelength of the light passing through the exit slit will vary throughout the range of the operating spectrum of the instrument. The wavelength transmitted through the exit slit at any given position of the grating will normally comprise a first order or primary center wavelength as well as second order and higher order wavelengths. This fact is used advantageously in some instruments to expand the wavelength band of the instrument, but it is also a problem because for analysis purposes, it is required that only a very narrow wavelength band of light be presented by the instrument. In addition, some stray light from the entrance slit will reflect off of other surfaces in the grating chamber and pass through the exit slit. To solve these problems, the instruments in the prior art and in current use employ order sorting filters to filter out the undesired wavelengths from the transmitted light. For example, in a near infrared instrument, the spectrum of interest usually ranges between 1100 nanometers and 2600 nanometers. When the grating of such an instrument is positioned so that it is transmitting light through the exit silt in the range of 2200-2600 nanometers of first order light, it will also be transmitting second order light in the range of 1100-1300 nanometers. Also, when the grating is in position to transmit light from 2200-1100 nanometers, it will also be transmitting second order light, outside the range of interest, from 1100 nanometers down to 550 nanometers. Order sorting filters are employed to filter out this unwanted light as well as to reduce the stray light passing through the exit slit. Because some of the second order light occurs within the band of interest, the order sorting filter must be changed to different positions of the grating. For an instrument employing a high speed oscillating grating, the filter must be changed at a rapid grate in synchronism with the grating oscillation. The need for the order sorting filters thus increases the complexity and cost of the instrument. Moreover, the order sorting filters are a source of attenuation and interference distortion of the light transmitted through the exit slit.